Known techniques for charging a toner include triboelectric charging, ion irradiation, electrostatic induction utilizing conductivity, charge intoduction, and the like. Among them, the triboelectric charging is the most convenient and broadly adopted. Developers for the triboelectric charging technique include a two-component system, including a mixture of a toner and carrier particles charged to the opposite polarity of the toner, charge and a one-component system utilizing static charging between a charging blade or a charging roll, etc. and a toner or charging of toner particles.
According to this technique, charge control is effected by addition of an electron donating or attracting substance to the toner or the charge-imparting material, such as a carrier and a blade. The triboelectric charging phenomenon is complicated, and its mechanism has not been completely elucidated. In general, an electron donating substance is positively charged, while an electron attracting substance is negatively charged.
More specifically, charge control has been conventionally carried out by (i) using charge control agents, such as dyes, pigments, surface active agents, inorganic powders, etc. mixed with the toner or adhered onto the toner surface, or (ii) using resinous materials having a functional group, such as --COOH, --CN, a halogen (e.g., --Cl), --NO.sub.2, --NH.sub.2, etc., as a binder.
However, use of the above-described conventional charge control agents has not fully satisfied the requirements on charge control necessary to provide charge stability against the passage of time and environmental changes.
Further, a colorless or pale-colored charge control agent which can be used satisfactorily for a color developer has not yet been developed.
Frequently, an organic and/or inorganic powder in a particulate, fibrous or plate-like form is incorporated into the inside of toner particles or adhered onto the outer surface of toner particles for various purposes, not limited to charge control, such as improvement of powder fluidity or the anti-blocking properties of a toner, improvement of cleanability of a toner on the surface of a toner image-receiving element, e.g., a photoreceptor, control of rheological characteristics of a toner per se, and the like. In these cases, too, since the inorganic or organic powder added naturally influences the electrical characteristics of a toner, including charging properties, not only the electrical characteristics of the powder per se but also the compatibility between the powder and the toner are of great importance.
In the case of a negatively charging toner, silica or other fine powders are frequently added. However, addition of such silica or other fine powders to a positively charging toner deteriorates the charging properties of the toner. In order to overcome this problem, it has been proposed to use a silica or other fine powder subjected to surface treatment with positively charging compounds containing an amino group or a modified amino group to effect charge control, as disclosed in Japanese Patent Publication No. 22447/78 and Japanese Patent Application (OPI) Nos. 79254/83, 43456/84, and 189352/84 (the term "OPI" as used herein means "unexamined published Japanese Patent application").
While these techniques provide a limited improvement, the toner still has problems of stability over time, environmental stability, and physical and chemical stability when used for repeated copying. An additive for a positively charging toner which satisfies both non-electrical requirements and electrical requirements has not yet been developed.